Molded component parts are commonly formed using sheet molding compounds or compositions (SMC) for automotive and building construction and finishing applications. For example, reinforced plastic doors formed by compression molding sheet molding compounds offer several advantages and attributes over typical wood or steel door in that they do not crack or warp over time, they withstand adverse weather, they do not dent easily, and they are generally lower in maintenance. Typically, such doors are molded and assembled with a wood frame and a polyurethane foam core of SMC.
Sheet molding compositions typically include unsaturated polyester or vinyl-ester resins, thickening agents, and a monomer such as styrene to copolymerize the unsaturated polyester during curing in the molding process. Typically, sheet molding compositions contain 20–25% by weight glass fiber reinforcement and 10–40% by weight of an inert filler material, e.g., calcium carbonate. Additionally, sheet molding compositions commonly contain thermoplastic polymers which are added to prevent undesirable shrinkage.
Initially, the sheet molding composition has a viscosity of about 20,000 to 40,000 centipoise, i.e., low viscosity, sticky liquids. This low viscosity provides good wetting of the glass fibers. However, the sheet molding compositions must be brought to a significantly higher viscosity to ensure ease of handling and proper flow of the composition into heated molds for curing during the molding process. Typically, the thickening or maturation process includes mixing the polyester resin with thickening agents such as alkali metal oxides and hydroxides that react or complex with the polyester resin. The maturation time is ordinarily from 3 to 5 days to reach the desired molding viscosity. During that time, the viscosity of the sheet molding composition increases such that the material may be handled by the mold operators. At this point, the viscosity is approximately 25–60 million centipoise. However, over time, the sheet molding composition continues to thicken and therefore has a limited shelf life, usually only 10 to 14 days from manufacturing. If the sheet molding composition is not used within this time, the viscosity increases to a point that the sheet molding composition cannot be used and must be discarded.
In compression molding, a hydraulic press provides high pressure and elevated temperatures to soften the hardened sheet molding composition so that it can flow into the mold cavity. However, because of the extremely high pressures associated with compression molding, e.g., 1,000–2,000 psi, the dies must be formed of forged tool steel, which is very expensive. Therefore, a large capital investment is required for the press and molds.
Additionally, in the past, reduced viscosity sheet molding compounds with appropriate handling characteristics have been achieved by using either crystalline polyesters or isocyanates to impart the thickening of the compound. Crystalline polyesters are heated and applied to the sheet molding compound paste formulation. After they have cooled, they increase the viscosity of the molding compound to a level where handling and processing is possible.
Once placed in the heated mold, the crystalline polyesters return to their reduced viscosity state, thereby rendering a low viscosity effect of the sheet molding compound. These crystalline polyesters are typically thermoplastic and, as such, can act as a low profile agent. However, crystalline polyesters are typically quite expensive compared to traditional thickening agents. Further, the necessary heating and controlled cooling of the SMC paste can lead to an increased demand for capital in the compounding operation, difficulty in wetting out the reinforcement it the SMC cools too rapidly, and other issues related to the inability of the SMC paste to evenly distribute the reinforcing fibers during the flow portion of molding if the hot viscosity of the paste is not enough to move the fibers.
In the case of isocyanate thickening of a sheet molding compound, isocyanates such as methylene diisocyanate or toluene diisocyanate are added to an SMC formulation in the last moment prior to compounding. The isocyanate groups bond to the hydroxide end groups of the polyester to form a urethane cross link to effectively thicken the SMC. In the case of a pure, unmodified vinyl-ester based SMC, the addition of a glycol or polyol group may be necessary to effectively thicken the sheet molding compound. While this urethane linkage tends to toughen the molded SMC part and may act as an additional low shrink additive, it has several detracting features. First, shelf life on a sheet molding compound thickened with isocyonate or a polyol group is typically less than 10 days from time of compounding. Secondly, isocyanates cause a sensitizing reaction in humans. Therefore, they must be strictly contained during storage and mixing and high levels of ventilation must be used during compounding in order to prevent allergic type symptoms including rash or skin outbreaks or difficulty or stoppage of breathing. Even with such measures taken and strictly adhered to, some individuals still will not be able to work around compounds containing these chemicals due to their hypersensitivity.
It is therefore desirable to provide a method for controlling the flow characteristics of the sheet molding compound to a lower viscosity which can be used in low pressure molding conditions such that the disadvantages of the prior art are overcome.